As winter approaches, your greenhouse is likely to feel the chill. While it can be difficult to maintain the warmth of spring or summer without using significant energy, there’s still plenty of gardening you can do, and your greenhouse provides opportunities that wouldn’t be possible outdoors.

Frost Protection: Heating or Insulation

Your primary concern in winter is preventing frost, and you can do this by either heating or insulating your greenhouse. Most gardeners use a combination of both to keep the warmth inside when using a heater.

Tip 1: Always monitor the temperature. Invest in a reliable thermometer (or multiple) to keep an eye on your greenhouse conditions.

Heating Your Greenhouse

Heating your greenhouse is the most effective way to ensure a stable growing environment throughout the year. While it won’t reach summer temperatures, heated spaces offer far more growing potential than unheated ones. You can explore DIY solutions like a wood-burning stove or underfloor heating, but a dedicated greenhouse heater is often the simplest option. Heated propagators and mats also provide focused warmth without heating the entire space.

Choosing the Right Heater

  • Gas Heaters (Paraffin or Propane): These don’t require electricity but may increase humidity, leading to mould or pests, and carry a higher fire risk.
  • Oil Heaters: Similar to gas heaters, these operate without electricity but come with moisture and safety concerns.
  • Electric Heaters: The safest option, providing dry heat to discourage fungal growth, but they need a mains power supply.
  • Solar-Powered Heaters: Eco-friendly but less efficient. Research thoroughly before choosing one.

Do's and Don’ts with Heaters

  • Ensure proper air circulation around the heater to protect plants and reduce fire risk.
  • Regularly ventilate the space to avoid stagnant air, which can harm plants and encourage mould.
  • Invest in a heater with a thermostat to conserve energy.
  • If using gas or oil heaters, install safety devices like smoke alarms or carbon monoxide detectors.

Tip 2: If your greenhouse has partitions, only heat the section you need to save energy and reduce costs.

Alternative Heating and Insulation Methods

Many gardeners avoid heating due to the cost and environmental impact, but there are passive ways to retain warmth:

  • Bubble Wrap: Line the walls and containers with horticultural bubble wrap for better insulation and light transmission.
  • Fleece: Cover plants with fleece during cold nights, but remove it on sunny days. Hanging fleece in larger greenhouses can lower the ceiling and conserve heat.
  • Polystyrene: Use polystyrene under pots to keep them off the cold ground and retain warmth.
  • Reflective Materials: Foil panels, mirrors, or even fun items like disco balls can reflect light and warmth around the space.
  • Heat Sinks: Large black water bottles, raised beds of brick, or heat-conductive materials like pea gravel or metal pieces can absorb and release heat.
  • Composting: Composting inside the greenhouse generates heat during decomposition, offering warmth while creating nutrient-rich compost.

By using a combination of heating, insulation, or both, you can keep your greenhouse frost-free and extend your growing season through the winter months.